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Fracture behavior of metallic glass thin films

Focus: Microcantilever fracture tests were carried out on various metallic glass thin films systems to evaluate their fracture strength and fracture toughness as a function of Poisson’s ratio.

The fracture energy of bulk metallic glasses are known to scale with Poisson’s ratio. Ab-initio calculations predicted neat theoretical strengths in several new metallic glass compositions. Thin films of these compositions were sputter deposited and their fracture properties evaluated at the micron-scale using cantilever bending. A brittle to ductile transition in fracture toughness was observed with increasing Poisson’s ratio while fracture strengths close to theoretical values were recorded from bend tests.

Both linear elastic fracture mechanics in terms of KIC and elastic-plastic fracture mechanics using J-integral was employed for the calculations. The fracture surfaces were observed using HRSEM to determine the critical meniscus spacing of the dimples.

Compositionally graded thin film metallic glasses with continuously varying elastic properties will also be explored for their fracture properties.

Fig Caption: (a) Stress vs displacement curve obtained from microcantilever fracture experiments of CoB and PdAlCuY metallic glass thin films, showing two extreme ends of ductile and brittle fracture. (b) Experimentally determined fracture toughness values obtained from metallic glass thin films superimposed on BMG fracture energy values, as a function of computed Poisson’s ratio.
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